Liquid-crystalline medium

ABSTRACT

The invention relates to a liquid-crystalline medium which is characterized in that it comprises one or more compounds of the formula A  
                 
 
     and one or more compounds of the formula B  
                 
 
     in which R a , R b , R bb , x and L 1  are as defined in claim 1.

[0001] The invention relates to a liquid-crystalline medium and totwisted nematic (TN) and supertwisted nematic (STN) liquid-crystaldisplays having very short response times and good steepnesses and angledependencies, and to the novel nematic liquid-crystal mixtures usedtherein.

[0002] TN displays are known, for example from M. Schadt and W.Helfrich, Appl. Phys. Lett., 18, 127 (1971). STN displays are known, forexample from EP 0 131 216 B1; DE 34 23 993 A1; EP 0 098 070 A2; M.Schadt and F. Leenhouts, 17th Freiburg Congress on Liquid Crystals(8.-i0.04.87); K. Kawasaki et al., SID 87 Digest 391 (20.6); M. Schadtand F. Leenhouts, SID 87 Digest 372 (20.1); K. Katoh et al., JapaneseJournal of Applied Physics, Vol. 26, No. 11, L 1784-L 1786 (1987); F.Leenhouts et al., Appl. Phys. Lett. 50 (21), 1468 (1987); H. A. vanSprang and H. G. Koopman, J. Appl. Phys. 62 (5), 1734 (1987); T. J.Scheffer and J. Nehring, Appl. Phys. Lett. 45 (10), 1021 (1984), M.Schadt and F. Leenhouts, Appl. Phys. Lett. 50 (5), 236 (1987) and E. P.Raynes, Mol. Cryst. Liq. Cryst. Letters Vol. 4 (1), pp. 1-8 (1986). Theterm STN here covers any relatively highly twisted display elementhaving a twist angle with a value of between 160° and 360°, such as, forexample, the display elements according to Waters et al. (C. M. Waterset al., Proc. Soc. nf. Disp. (New York) (1985) (3rd Intern. DisplayConference, Kobe, Japan), STN-LCDs (DE-A 35 03 259), SBE-LCDs (T. J.Scheffer and J. Nehring, Appl. Phys. Lett. 45 (1984) 1021), OMI-LCDs (M.Schadt and F. Leenhouts, Appl. Phys. Lett. 50 (1987), 236, DST-LCDs(EP-A 0 246 842) or BW-STN-LCDs (K. Kawasaki et al., SID 87 Digest 391(20.6)).

[0003] STN displays are distinguished compared with standard TN displaysby significantly better steepnesses of the electro-opticalcharacteristic line and, associated therewith, better contrast values,and by significantly lower angle dependence of the contrast.

[0004] Of particular interest are TN and STN displays having very shortresponse times, in particular also at relatively low temperatures. Inorder to achieve short response times, the rotational viscosities of theliquid-crystal mixtures have hitherto been optimized using mostlymonotropic additives having relatively high vapor pressure. However, theresponse times achieved were not adequate for every application.

[0005] In order to achieve a steep electro-optical characteristic linein the displays according to the invention, the liquid-crystal mixturesshould have relatively large values for the ratio between the elasticconstants K₃₃/K₁₁ and relatively small values for Δε/ε_(⊥), where Δε isthe dielectric anisotropy and ε^(⊥) is the dielectric constantperpendicular to the longitudinal molecular axis.

[0006] In addition to optimization of the contrast and response times,further important requirements are made of mixtures of this type:

[0007] 1. broad d/p window

[0008] 2. high long-term chemical stability

[0009] 3. high electrical resistance

[0010] 4. low frequency and temperature dependence of the thresholdvoltage.

[0011] The parameter combinations achieved are still far from adequate,in particular for high-multiplex STN displays (with a multiplex rate inthe region of about 1/400), but also for medium- and low-multiplex STNdisplays (with multiplex rates in the region of about 1/64 and 1/16respectively), and TN displays. This is partly attributable to the factthat the various requirements are affected in opposite manners bymaterial parameters.

[0012] Thus, there continues to be a great demand for TN and STNdisplays, in particular for medium- and low-multiplex STN displays,having very short response times at the same time as a largeworking-temperature range, high characteristic-line steepness, goodangle dependence of the contrast and low threshold voltage which meetthe above-mentioned requirements.

[0013] The invention has an object of providing liquid-crystallinemedia, in particular for TN and STN displays, which do not have theabove-mentioned disadvantages or only do so to a lesser extent and atthe same time have short response times, in particular at lowtemperatures, and very good steepnesses. Upon further study of thespecification and appended claims, further objects and advantages ofthis invention will become apparent to those skilled in the art.

[0014] It has now been found that these and other objects can beachieved if use is made of liquid-crystal mixtures which comprise one ormore compounds of the formula A

[0015] and at least one compound of the formula B

[0016] in which

[0017] R^(a) and R^(bb) are H, an alkyl radical having from 1 to 12carbon atoms which is unsubstituted, monosubstituted by CN or CF₃ or atleast monosubstituted (i.e., up to perhalosubstituted) by halogen, italso being possible for one or more CH₂ groups in these radicals to bereplaced, in each case independently of one another, by

[0018]  in such a way that O atoms are not linked directly to oneanother,

[0019] R^(b) is an alkenyl radical having from 2 to 7 carbon atoms,

[0020] L¹ is H or F, and

[0021] x is 0 or 1.

[0022] The use of the compounds of the formulae A and B in the mixturesfor TN and STN displays according to the invention results in

[0023] high steepness of the electro-optical characteristic line,

[0024] low temperature dependence of the threshold voltage,

[0025] very fast response times, in particular at low temperatures.

[0026] The compounds of the formulae A and B significantly shorten, inparticular, the response times of TN and STN mixtures whilesimultaneously increasing the steepness and reducing the temperaturedependence of the threshold voltage.

[0027] The mixtures according to the invention are furthermoredistinguished by the following advantages:

[0028] they have low viscosity,

[0029] they have a low threshold voltage and operating voltage, and

[0030] they effect long shelf lives in the display at low temperatures.

[0031] The invention furthermore relates to a liquid-crystal displayhaving

[0032] two outer plates, which, together with a frame, form a cell,

[0033] a nematic liquid-crystal mixture of positive dielectricanisotropy located in the cell,

[0034] electrode layers with alignment layers on the insides of theouter plates,

[0035] a tilt angle between the longitudinal axis of the molecules atthe surface of the outer plates and the outer plates of from 0 degree to30 degrees, and

[0036] a twist angle of the liquid-crystal mixture in the cell fromalignment layer to alignment layer with a value of between 22.5° and600°,

[0037] a nematic liquid-crystal mixture consisting of

[0038] a) 15-75% by weight of a liquid-crystalline component Aconsisting of one or more compounds having a dielectric anisotropy ofgreater than +1.5;

[0039] b) 25-85% by weight of a liquid-crystalline component Bconsisting of one or more compounds having a dielectric anisotropy ofbetween −1.5 and +1.5;

[0040] c) 0-20% by weight of a liquid-crystalline component D consistingof one or more compounds having a dielectric anisotropy of below −1.5,and

[0041] d) if desired, an optically active component C in such an amountthat the ratio between the layer thickness (separation of the outerplates) and the natural pitch of the chiral nematic liquid-crystalmixture is from about 0.2 to 1.3,

[0042] characterized in that component A comprises at least one compoundof the formula A

[0043] and component B comprises at least one compound of the formula B

[0044] in which R^(a), R^(b), R^(bb), x and L¹ are as defined above.

[0045] The invention also relates to TN and STN displays, in particularmedium- and low-multiplexed STN displays, containing the liquid-crystalmixture according to the invention.

[0046] Formula A covers, in particular, compounds of the sub-formulaeA-1 to A-4

[0047] in which alkyl is a straight-chain or branched alkyl radicalhaving from 1 to 12 carbon atoms, and alkenyl is a straight-chain orbranched alkenyl radical having from 2 to 12 carbon atoms.

[0048] Particular preference is given to mixtures according to theinvention which comprise at least one compound of the formula A-1 and/orA-2, particularly preferably in each case at least one compound of theformula A-1 in which L¹=F.

[0049] In the formulae A, R^(a) (and alkyl or alkenyl in formulae A-1 toA-4) is particularly preferably straight-chain alkyl or alkoxy,1E-alkenyl or 3E-alkenyl having from 2 to 7 carbon atoms.

[0050] Formula B covers, in particular, compounds of the sub-formulaeB-1 to B-6

[0051] in which

[0052] alkyl are each, independently of one another, a straight-chain orbranched alkyl radical having from 1 to 12 carbon atoms, and

[0053] alkenyl and

[0054] alkenyl* are each, independently of one another, a straight-chainor branched alkenyl radical having from 2 to 12 carbon atoms.

[0055] The radicals “alkyl” and “alkenyl” or “alkenyl*” are preferablystraight-chain and have up to 7 carbon atoms.

[0056] The use of compounds of the formulae A and B in theliquid-crystal mixtures according to the invention results inparticularly low rotational viscosity values and in TN and STN displayshaving high steepness and fast response times, in particular at lowtemperatures. For example, the threshold voltages of the inventive mediain STN-displays with 240° twist and 3° tilt are preferably in the rangefrom 1.0 V to 2.0 V, particularly preferably in the range from about 1.1V to 1.8 V. The values of the steepness of the electrooptical responseof the media in the STN-displays makes them suitable for medium havinghigher multiplex ratios. Typically they can be used by multiplex ratiosof up to 1/128 and even up to 1/200 or more and even up to 1/400.Preferably the media with the lower threshold voltages are used with amultiplex ratio of 1/64 up to 1/128. The low values of the rotationalviscosity of the media lead to fast response times. The response timesin the STN-displays, as mentioned, are preferably in the range fromabout 60 ms to 300 ms, when measured with a multiplex ratio of 1/64 anda bias of 1/9. More preferably, the response times are in the range from90 ms to 280 ms. The higher response times are observed for the mediawith the lower threshold voltages and vice versa.

[0057] Besides the compounds of the formula A, component A of theliquid-crystalline mixture according to the invention additionallypreferably comprises one or more 3,4,5-trifluorophenyl compoundsselected from the group consisting of the compounds of the formulae IIato III

[0058] Besides the compounds of the formulae A and B, the mediumaccording to the invention may additionally comprise one or morecompounds containing polar end groups, for example, those of theformulae II*a to II*s

[0059] in which R² is as defined for R^(a), and L³ and L⁴ are each,independently of one another, H or F. R² in these compounds isparticularly preferably alkyl, alkenyl, alkoxy or alkenyloxy having upto 7 carbon atoms.

[0060] The medium according to the invention or component A particularlypreferably comprises compounds of the formulae IIa, IIb, IIc, IId, IIe,IIf, IIg, IIj, II*b, II*c, II*d, II*f and/or II*i, in particularcompounds of the formulae IIa, IIb, IId, IIi, II*a and II*i. The mixtureaccording to the invention preferably further comprises one or morecyano compounds of the formulae IIIa to IIIj:

[0061] in which R³ is as defined for R^(a), and L¹, L² and L⁵ are each,independently of one another, H or F. R³ in these compounds isparticularly preferably alkyl, alkenyl or alkoxy having up to 7 carbonatoms.

[0062] Particular preference is given to mixtures which comprise one ormore compounds of the formulae IIIb, IIIc and IIIf, in particular thosein which L¹ and/or L² is F.

[0063] Preference is furthermore given to mixtures which comprise one ormore compounds of the formula IIIf and/or IIIg in which L² is H and L¹is H or F, in particular F.

[0064] The individual compounds of the formulae A, B and of the formulaeIIa-IIl, II*a-II*s and IIIa to IIIj and their sub-formulae or also othercompounds which can be used in the mixtures or TN and STN displaysaccording to the invention are either known or can be preparedanalogously to the known compounds.

[0065] The compounds of the formula B have low viscosities, inparticular low rotational viscosities, and low values for the ratio ofthe elastic constants K₃₃/K₁₁, and therefore result in short responsetimes in the displays according to the invention, while the presence ofcompounds of the formula A of high dielectric anisotropy, in particularin increased concentrations, causes a reduction in the viscosity.

[0066] Preferred liquid-crystal mixtures comprise one or more compoundsof component A, preferably in a proportion of from 15% to 75%,particularly preferably from 20% to 65%. These compounds have adielectric anisotropy Δε≧+3, in particular Δε≧+8, particularlypreferably Δε≧+12.

[0067] Further preferred mixtures comprise

[0068] one or more, in particular one or two, compounds of the formulaA,

[0069] one, two or three compounds of each of the formulae A-1 and A-3,

[0070] one or more, in particular two, three or four, compounds of theformula B,

[0071] one or more, in particular from two to five, compounds of theformulae IIIb, IIIc and/or IIIf.

[0072] Preferred liquid-crystal mixtures comprise one or more compoundsof component B, preferably from 25 to 85%. The compounds from group Bare distinguished, in particular, by their low values for the rotationalviscosity γ₁.

[0073] Component B preferably further comprises one or more compounds ofthe formula IV

[0074] in which

[0075] m is 0 or 1,

[0076] R⁴ is an alkenyl group having from 2 to 7 carbon atoms,

[0077] R⁵ is as defined for R^(a) in the case where m=0 and is F, Cl,CF₃ or OCF₃ in the case where m=1,

[0078] L¹ and L² are each, independently of one another, H or F.

[0079] Particularly preferred compounds of the formula IV are those inwhich R⁴ is alkenyl having from 2 to 7 carbon atoms, in particular thoseof the following formulae:

[0080] in which R^(3a) and R^(4a) are each, independently of oneanother, H, CH₃, C₂H₅ or n-C₃H₇, and alkyl is an alkyl group having from1 to 7 carbon atoms.

[0081] Particular preference is given to TN and STN displays accordingto the invention in which the liquid-crystal mixture comprises at leastone compound of the formula IV1 and/or IV3 in which R^(3a) and R^(4a)each have the same meaning, and displays in which the liquid-crystalmixture comprises at least one compound of the formula IV5.

[0082] In a further preferred embodiment, the mixtures according to theinvention comprise one or more compounds of the formula IV6.

[0083] Component B preferably furthermore comprises compounds selectedfrom the group consisting of the bicyclic compounds of the formulae V-1to V-9

[0084] and/or one or more compounds selected from the group consistingof the tetracyclic compounds of the formulae V-26 to V-32

[0085] in which R⁶ and R⁷ are as defined for R^(a) in the formula A, andL is H or F.

[0086] Particular preference is given to compounds of the formulae V-24to V-32 in which R⁶ is alkyl and R⁷ is alkyl or alkoxy, in particularalkoxy, each having from 1 to 7 carbon atoms. Preference is furthermoregiven to compounds of the formulae V-24, V-26 and V-32 in which L is F.

[0087] R⁶ and R⁷ in the compounds of the formulae V-1 to V-32 areparticularly preferably straight-chain alkyl or alkoxy having from 1 to12 carbon atoms.

[0088] If desired, the liquid-crystalline mixtures comprise an opticallyactive component C in such an amount that the ratio between the layerthickness (separation of the outer plates) and the natural pitch of thechiral nematic liquid-crystal mixture is greater than 0.2. Amultiplicity of chiral dopants, some of which are commerciallyavailable, is available to the person skilled in the art for thecomponent, such as, for example, cholesteryl nonanoate, S-811, S-1011and S-2011 from Merck KGaA, Darmstadt, and CB15 (BDH, Poole, UK). Thechoice of dopants is not crucial per se.

[0089] The proportion of the compounds of component C is preferably from0 to 10%, in particular from 0 to 5%, particularly preferably from 0 to3%.

[0090] The mixtures according to the invention may also, if desired,comprise up to 20% of one or more compounds having a dielectricanisotropy of less than −2 (component D).

[0091] If the mixtures comprise compounds of component D, these arepreferably one or more compounds containing the structural unit2,3-difluoro-1,4-phenylene, for example compounds as described in DE-A38 07 801, 38 07 861, 38 07 863, 38 07 864 or 38 07 908. Particularpreference is given to tolans containing this structural unit, asdescribed in the International Patent Application PCT/DE 88/00133.

[0092] Further known compounds of component D are, for example,derivatives of the 2,3-dicyanohydroquinones or cyclohexane derivativescontaining the structural unit

[0093] as described in DE-A 32 31 707 and DE-A 34 07 013.

[0094] The liquid-crystal displays according to the invention preferablycontain no compounds of component D.

[0095] The term “alkenyl” in the definition of R^(a), R^(b), R^(bb), R²,R³, R⁴, R⁵, R⁶ and R⁷ covers straight-chain and branched alkenyl groups,in particular the straight-chain groups. Particularly preferred alkenylgroups are C₂-C₇-1E-alkenyl, C₄-C₇-3E-alkenyl, C₅-C₇-4-alkenyl,C₆-C₇-5-alkenyl and C₇-6-alkenyl, in particular C₂-C₇-1E-alkenyl,C₄-C₇-3E-alkenyl and C₅-C₇-4-alkenyl.

[0096] Examples of preferred alkenyl groups are vinyl, 1E-propenyl,1E-butenyl, 1E-pentenyl, 1E-hexenyl, 1E-heptenyl, 3-butenyl,3E-pentenyl, 3E-hexenyl, 3E-heptenyl, 4-pentenyl, 4Z-hexenyl,4E-hexenyl, 4Z-heptenyl, 5-hexenyl, 6-heptenyl and the like. Groupshaving up to 5 carbon atoms are generally preferred.

[0097] Further preferred embodiments relate to liquid-crystal mixturesaccording to the invention which

[0098] additionally comprise one or more, particularly preferably one,two or three, heterocyclic compounds of the formula Va and/or Vb

[0099] in which

[0100] R⁶ and R⁷ are as defined above, and

[0101] Y is F or Cl,

[0102] where the proportion of the compounds from the group consistingof Va and Vb is preferably from 2 to 35%, in particular from 5 to 20%,

[0103] additionally comprise one or more, particularly preferably one,two or three, tolan compounds of the formulae T2a, T2b and/or T2c

[0104] in which R⁶ and R⁷ are as defined above.

[0105] The proportion of the compounds from the group consisting of T2a,T2b and/or T2c is preferably from 2 to 40%, in particular from 5 to 30%.The mixture according to the invention preferably comprises two or threecompounds of the formula T2a and/or T2b.

[0106] In particularly preferred embodiments, the mixtures comprise

[0107] at least one compound of the formula IIIc;

[0108] at least one tolan compound of the formula T2c;

[0109] at least two, in particular three, compounds of the formula B;

[0110] at least one compound of the formula T2a and/or at least onecompound of the formula T2b;

[0111] at least one compound of the formula

[0112] in which L¹ is H or F;

[0113] at least 2.5% by weight of the compounds of the formula IV5;

[0114] 5-30% by weight, preferably 10-25% by weight, of the compounds ofthe formula A;

[0115] 5-30% by weight, preferably 5-20% by weight, of the compounds ofthe formula B;

[0116] at least one compound of the formula A in which R^(a) is CH₃,C₂H₅, n-C₃H₇ or n-C₅H₁₁.

[0117] Further particularly preferred embodiments relate toliquid-crystal mixtures which comprise

[0118] a total of from three to six compounds of the formulae A and B,where the proportion of these compounds with respect to the mixture as awhole is from 25 to 65%, in particular from 30 to 55%,

[0119] more than 20% of compounds of positive dielectric anisotropy, inparticular having Δε≧+12.

[0120] The mixtures according to the invention are distinguished, inparticular on use in TN and STN displays of high layer thicknesses, byvery low total response times (t_(tot)=t_(on)+t_(off)).

[0121] The liquid-crystal mixtures used in the TN and STN cellsaccording to the invention are dielectrically positive, with Δε≧+1.Particular preference is given to liquid-crystal mixtures with Δε≧+3, inparticular with Δε≧+5.

[0122] The liquid-crystal mixtures according to the invention havefavorable values for the threshold voltage V_(10/0/20) and for therotational viscosity γ₁. If the value for the optical path differenced·Δn is pre-specified, the value for the layer thickness d is determinedby the optical anisotropy Δn. In particular at relatively high valuesfor d·Δn, the use of liquid-crystal mixtures according to the inventionhaving a relatively high value for the optical anisotropy is generallypreferred, since the value for d can then be selected to be relativelysmall, which results in more favorable values for the response times.However, liquid-crystal displays according to the invention whichcontain liquid-crystal mixtures according to the invention with smallervalues for Δn are also characterized by advantageous values for theresponse times.

[0123] The liquid-crystal mixtures according to the invention arefurthermore characterized by advantageous values for the steepness ofthe electro-optical characteristic line, and can be operated with highmultiplex rates, in particular at temperatures above 20° C. In addition,the liquid-crystal mixtures according to the invention have highstability and favorable values for the electrical resistance and thefrequency dependence of the threshold voltage. The liquid-crystaldisplays according to the invention have a large working-temperaturerange and good angle dependence of the contrast.

[0124] The construction of the liquid-crystal display elements accordingto the invention from polarizers, electrode base plates and electrodeshaving a surface treatment such that the preferential alignment(director) of the liquid-crystal molecules in each case adjacent theretois usually twisted by a value of from 160° to 720° from one electrode tothe other corresponds to the usual structure for display elements ofthis type. The term “usual structure” here is broadly drawn and alsocovers all derivatives and modifications of the TN and STN cell, inparticular also matrix display elements and display elements containingadditional magnets.

[0125] The surface tilt angle at the two outer plates may be identicalor different. Identical tilt angles are preferred. Preferred TN displayshave pre-tilt angles between the longitudinal axis of the molecules atthe surface of the outer plates and the outer plates of from 0° to 7°,preferably from 0.01° to 5°, in particular from 0.1 to 2°. In the STNdisplays, the pre-tilt angle is from 1° to 30°, preferably from 1° to12° and in particular from 3° to 10°.

[0126] The twist angle of the TN mixture in the cell has a value ofbetween 22.5° and 170°, preferably between 45° and 130° and inparticular between 80° and 115°. The twist angle of the STN mixture inthe cell from alignment layer to alignment layer has a value of between100° and 600°, preferably between 170° and 300° and in particularbetween 180° and 270°.

[0127] The liquid-crystal mixtures which can be used in accordance withthe invention are prepared in a manner which is conventional per se. Ingeneral, the desired amount of the components used in lesser amount aredissolved in the components making up the principal constituent,advantageously at elevated temperature. It is also possible to mixsolutions of the components in an organic solvent, for example inacetone, chloroform or methanol, and to remove the solvent again, forexample by distillation, after thorough mixing.

[0128] The dielectrics may also comprise further additives which areknown to the person skilled in the art and are described in theliterature. For example, 0-15% of pleochroic dyes may be added.

[0129] The entire disclosures of all applications, patents andpublications, cited above or below, and of corresponding Germanapplication No. 101 63 271.1, filed Dec. 21, 2001, is herebyincorporated by reference.

[0130] In the present application and in the examples below, thestructures of the liquid-crystal compounds are indicated by means ofacronyms, the transformation into chemical formulae taking place inaccordance with Tables A and B. All radicals C_(n)H_(2n+1) andC_(m)H_(2m+1) are straight-chain alkyl radicals having n and m carbonatoms respectively (n and m: 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11 or12). The alkenyl radicals have the trans-configuration. The coding inTable B is self-evident. In Table A, only the acronym for the parentstructure is indicated. In individual cases, the acronym for the parentstructure is followed, separated by a dash, by the code indicated in thetable below for the substituents R¹*, R²*, L¹*, L²* and L³*. Code forR^(1*), R^(2*), L^(1*L ,L) ^(2*),L^(3*) R^(1*) R^(2*) L^(1*) L^(2*)L^(3*) nm C_(n)H_(2n+1) C_(m)H_(2m+1) H H H nOm OC_(n)H_(2n+1)C_(m)H_(2m+1) H H H nO.m C_(n)H_(2n+1) OC_(m)H_(2m+1) H H H nC_(n)H_(2n+1) CN H H H nN.F C_(n)H_(2n+1) CN H H F nN.F.F C_(n)H_(2n+1)CN H F F nF C_(n)H_(2n+1) F H H H nOF OC_(n)H_(2n+1) F H H H nF.FC_(n)H_(2n+1) F H H F nmF C_(n)H_(2n+1) C_(m)H_(2m+1) F H H nOCF₃C_(n)H_(2n+1) OCF₃ H H H n-Vm C_(n)H_(2n+1) —CH═CH—C_(m)H_(2m+1) H H HnV-Vm C_(n)H_(2n+1—CH═CH—) —CH═CH—C_(m)H_(2m+1) H H H

[0131] The TN and STN displays preferably contain liquid-crystallinemixtures composed of one or more compounds from Tables A and B. TABLE A(L¹*, L²*, L³* = H or F)

BCH

CBC

CH

CCP

CPTP

PTP

ECCP

EPCH

CP

ME

HP

PCH

CCPC

CCH

[0132] TABLE B

CBC-nmF

CP-nmF

CCZU-n-F

CC-nV-Vm

CC-n-V

CCP-Vn-m

CCG-V-F

CCP-nV-m

PYP-nF

PYP-nm

PTP-nOm

CDU-n-F

PPTUI-n-m

CVCP-Vn-0m

PGU-n-F

CGU-n-F

CVCP-V-(0)n

CVCP-V-0n

PZU-V2-N

CVCP-1V-0n

Du-n-N

PCH-nN.F.F

[0133] TABLE C Table C shows dopants which are usually employed in themixtures according to the invention:

C 15

CB 15

CM 21

R/S-811

CM 44

CM 45

CM 47

CN

R/S-2011

R/S-1011

R/S-3011

[0134] TABLE D Stabilizers which can be added, for example, to themixtures according to the invention are shown below:

EXAMPLES

[0135] The following examples are intended to illustrate the inventionwithout representing a limitation. The following abbreviations are used:

[0136] cl.p. clearing point (nematic-isotropic phase transitiontemperature),

[0137] S-N smectic-nematic phase transition temperature,

[0138] visc. flow viscosity (mm²/S, unless stated otherwise, at 20° C.),

[0139] Δn optical anisotropy (589 nm, 20° C.)

[0140] Δε dielectric anisotropy (1 kHz, 20° C.)

[0141] γ₁ rotational viscosity (mPa·s at 20° C.)

[0142] steep characteristic line steepness=(V₉₀/V₁₀−1)·100 [%]

[0143] V₁₀ threshold voltage=characteristic voltage at a relativecontrast of 10%,

[0144] V₉₀ characteristic voltage at a relative contrast of 90%,

[0145] t_(ave)$\frac{t_{on} + t_{off}}{2}\left( {{average}\quad {response}\quad {time}} \right)$

[0146] t_(on) time from switching on until 90% of the maximum contrastis reached, t_(off) time from switching off until 10% of the maximumcontrast is reached,

[0147] mux multiplex rate

[0148] t_(store) low temperature storage stability in hours (−20° C.,−30° C., −40° C.)

[0149] Above and below, all temperatures are given in ° C. Thepercentages are percent by weight. All values relate to 20° C., unlessstated otherwise. The displays are addressed, unless stated otherwise,without multiplexing. The twist is 240°, unless stated otherwise.Example 1 ME2N.F  7.00% Clearing point [° C.]: +91.0 ME3N.F  7.00% Δn[589 nm, 20° C.]: +0.1393 ME4N.F  7.00% Mux: {fraction (1/64)}PCH-3N.F.F 10.00% Bias: {fraction (1/9 )} CCG-V-F 14.00% HTP [1/μm, 20°C.]: −12.06 CC-5-V  3.00% d · Δn [μm, 20° C.]: 0.85 CCP-V-1 13.00% Twist[°]: 240 CCP-V2-1  7.00% V_(10.0,20) [V]: 1.19 PPTUI-3-2 10.00% t_(ave)[ms, 20° C.] 250 CCPC-33  4.00% CCPC-34  4.00% CCPC-35  4.00% DU-3-N10.00% Example 2 ME2N.F  6.00% Clearing point [° C.]: +94.0 ME3N.F 6.00% Δn [589 nm, 20° C.]: +0.1392 ME4N.F  7.00% Mux: {fraction (1/64)}PCH-3N.F.F 10.00% Bias: {fraction (1/9 )} CCG-V-F 15.50% HTP [1/μm, 20°C.]: −11.99 CC-5-V  3.00% d · Δn [μm, 20° C.]: 0.85 CCP-V-1 13.00% Twist[°]: 240 CCP-V2-1  7.00% V_(10.0,20) [V]: 1.24 PPTUI-32 10.50% t_(ave)[ms, 20° C.] 240 CCPC-33  4.00% CCPC-34  4.00% CCPC-35  4.00% DU-3-N10.00% Example 3 ME2N.F  2.00% Clearing point [° C.]: +93.0 ME3N.F 3.00% Δn [589 nm, 20° C.]: +0.1371 PCH-3N.F.F 10.00% Mux: {fraction(1/64)} CCG-V-F 14.00% Bias: {fraction (1/9 )} CC-5-V  8.00% HTP [1/μm,20° C.]: −11.65 CCP-V-1 13.00% d · Δn [μm, 20° C.]: 0.85 CCP-V2-1  7.00%Twist [°]: 240 PPTUI-3-2 12.00% V_(10.0,20) [V]: 1.26 CCPC-33  4.00%t_(ave) [ms, 20° C.] 200 CCPC-34  4.00% CCPC-35  3.00% DU-3-N 10.00%PZU-V2-N 10.00% Example 4 PCH-3N.F.F 10.00% Clearing point [° C.]: +91.0ME2N.F  2.00% Δn [589 nm, 20° C.]: +0.1625 ME3N.F  2.00% Mux: {fraction(1/64)} CC-5-V  9.00% Bias: {fraction (1/9 )} CCG-V-F 18.00% HTP [1/μm,20° C.]: −11.37 CCP-V-1  8.00% d · Δn [μm, 20° C.]: 0.85 CCP-V2-1  5.00%Twist [°]: 240 CVCP-V-1  4.00% V_(10.0,20) [V]: 1.65 CVCP-V-O1  4.00%t_(ave) [ms, 20° C.] 110 CVCP-1V-01  2.00% PTP-102  3.00% PTP-201  3.00%PPTUI-3-2 20.00% DU-3-N 10.00% Example 5 PCH-3N.F.F 10.00% Clearingpoint [° C.]: +88.5 ME2N.F  1.00% Δn [589 nm, 20° C.]: +0.1608 ME3N.F 1.50% Mux: {fraction (1/64)} CC-5-V 10.00% Bias: {fraction (1/9 )}CCG-V-F 18.00% HTP [1/μm, 20° C.]: −11.13 CCP-V-1  8.00% d · Δn [μm, 20°C.]: 0.85 CCP-V2-1  5.00% Twist [°]: 240 CVCP-V-1  4.50% V_(10.0,20)[V]: 1.70 CVCP-V-01  3.00% t_(ave) [ms, 20° C.] 106 CVCP-1V-01  2.00%PTP-102  4.00% PTP-201  5.00% PPTUI-3-2 18.00% DU-3-N 10.00% Example 6CC-5-V 13.00% Clearing point [° C.]: +89.0 CCG-V-F 16.00% Δn [589 nm,20° C.]: +0.1596 CCP-V-1 10.00% Mux: {fraction (1/64)} CCP-V2-1  6.00%Bias: {fraction (1/9 )} CVCP-V- 1  4.00% HTP [1/μm, 20° C.]: −10.53CVCP-V-01  4.00% d · Δn [μm, 20° C.]: 0.85 CVCP-1V-01  2.00% Twist [°]:240 PTP-102  5.00% V_(10.0,20) [V]: 1.70 PTP-201  5.00% t_(ave) [ms, 20°C.] 95 PTP-301  4.50% PPTUI-3-2 12.00% DU-3-N 10.00% PZU-V2-N 10.00%Example 7 ME2N.F  2.00% Clearing point [° C.]: +91.0 ME3N.F  2.00% Δn[589 nm, 20° C.]: +0.1609 ME4N.F  7.00% Mux: {fraction (1/64)} CC-5-V11.00% Bias: {fraction (1/9 )} CCG-V-F 19.00% HTP [1/μm, 20° C.]: −10.91CCP-V-1  9.00% d · Δn [μm, 20° C.]: 0.85 CCP-V2-1  5.00% Twist [°]: 240CVCP-V-1  4.00% V_(10.0,20) [V]: 1.66 CVCP-V-01  4.00% CVCP-1V-01  2.00%PTP-102  5.00% PTP-201  3.00% PTP-302  3.00% PPTUI-3-2 14.00% DU-3-N10.00%

[0150] The entire disclosures of all applications, patents andpublications, cited herein and of corresponding German Patentapplication No. 101 63 271.1, filed Dec. 21, 2001, are incorporated byreference herein.

[0151] The preceding examples can be repeated with similar success bysubstituting the generically or specifically described reactants and/oroperating conditions of this invention for those used in the precedingexamples.

[0152] From the foregoing description, one skilled in the art can easilyascertain the essential characteristics of this invention and, withoutdeparting from the spirit and scope thereof, can make various changesand modifications of the invention to adapt it to various usages andconditions.

We claim:
 1. A liquid-crystalline medium, which comprises one or morecompounds of the formula A:

and at least one compound of the formula B:

in which R^(a) and R^(bb) are, independently, H, an alkyl radical havingfrom 1 to 12 carbon atoms which is unsubstituted, monosubstituted by CNor CF₃ or monosubstituted to perhalosubstituted by halogen, optionallyone or more CH₂ groups in these radicals being replaced, in each caseindependently of one another, by

 in such a way that O atoms are not linked directly to one another,R^(b) is an alkenyl radical having from 2 to 7 carbon atoms, L¹ is H orF, and x is 0 or
 1. 2. A liquid-crystalline medium according to claim 1,wherein at least one compound of the formula A is a compound selectedfrom the group consisting of compounds of the formulae A-1 to A-4:

in which alkyl is a straight-chain or branched alkyl radical having from1 to 12 carbon atoms, and alkenyl is a straight-chain or branchedalkenyl radical having from 2 to 12 carbon atoms.
 3. Aliquid-crystalline medium according to claim 1, which additionallycomprises at least one compound of one of the formulae IIa to IIl:

in which ² is H, an alkyl radical having from 1 to 12 carbon atoms whichis unsubstituted, monosubstituted by CN or CF₃ or monosubstituted toperhalosubstituted by halogen, optionally, one or more CH₂ groups inthese radicals being replaced, in each case independently of oneanother, by —O—, —S—,

—CH═CH—, —C≡C—, —CO—, —CO—O—, —O—CO— or —O—CO—O— in such a way that Oatoms are not linked directly to one another.
 4. A liquid-crystallinemedium according to claim 1, which additionally comprises one or morecyano compounds of the formulae IIIa to IIIj:

in which R³ is H, an alkyl radical having from 1 to 12 carbon atomswhich is unsubstituted, monosubstituted by CN or CF₃ or monosubstitutedto perhalosubstituted by halogen, optionally, one or more CH₂ groups inthese radicals being replaced, in each case independently of oneanother, by

 in such a way that O atoms are not linked directly to one another, andL¹, L² and L⁵ are each, independently of one another, H or F.
 5. Aliquid-crystalline medium according to claim 1, which additionallycomprises one or more compounds of the formula IV:

in which m is 0 or 1, R⁴ is an alkenyl group having from 2 to 7 carbonatoms, R⁵ in the case where m=0, is H, an alkyl radical having from 1 to12 carbon atoms which is unsubstituted, monosubstituted by CN or CF₃ ormonosubstituted to perhalosubstituted by halogen, optionally, one ormore CH₂ groups in these radicals being replaced, in each caseindependently of one another, by

 in such a way that O atoms are not linked directly to one another, and In the case where m=1, is F, Cl, CF₃ or OCF₃ L¹ and L² are each,independently of one another, H or F.
 6. A liquid-crystalline mediumaccording to claim 1, which additionally comprises one or more tolancompounds of the formulae T2a, T2b and/or T2c:

in which R⁶ and R⁷ are an alkyl radical having from 1 to 12 carbon atomswhich is unsubstituted, monosubstituted by CN or CF₃ or monosubstitutedto perhalosubstituted by halogen, optionally one or more CH₂ groups inthese radicals being replaced, in each case independently of oneanother, by

 in such a way that O atoms are not linked directly to one another.
 7. Aliquid-crystalline medium according to claim 1, wherein the mediumcomprises 5-40% by weight of compounds of the formula A.
 8. Aliquid-crystalline medium according to claim 1, wherein the mediumcomprises 5-30% by weight of compounds of the formula B.
 9. Aliquid-crystalline medium according to claim 7, wherein the mediumcomprises 5-30% by weight of compounds of the formula B.
 10. Aliquid-crystalline medium according to claim 1, which comprises morethan 20% by weight of compounds having a dielectric anisotropy, Δε, of≧+12.
 11. An electro-optical liquid-crystal display containing aliquid-crystalline medium according to claim
 1. 12. A TN or STNliquid-crystal display having two outer plates, which, together with aframe, form a cell, a nematic liquid-crystal mixture of positivedielectric anisotropy located in the cell, electrode layers withalignment layers on the insides of the outer plates, a tilt anglebetween the longitudinal axis of the molecules at the surface of theouter plates and the outer plates of from 0 degree to 30 degrees, and atwist angle of the liquid-crystal mixture in the cell from alignmentlayer to alignment layer with a value of between 22.5° and 600°, whereinthe nematic liquid-crystal mixture consists of a) 15-75% by weight of aliquid-crystalline component A consisting of one or more compoundshaving a dielectric anisotropy of greater than +1.5; b) 25-85% by weightof a liquid-crystalline component B consisting of one or more compoundshaving a dielectric anisotropy of between −1.5 and +1.5; c) 0-20% byweight of a liquid-crystalline component D consisting of one or morecompounds having a dielectric anisotropy of below −1.5, and d)optionally, an optically active component C in such an amount that theratio between the layer thickness, by separation of the outer plates,and the natural pitch of the chiral nematic liquid-crystal mixture isfrom about 0.2 to 1.3, wherein component A comprises at least onecompound of the formula A

and component B comprises at least one compound of the formula B

in which R^(a) and R^(bb) are H, an alkyl radical having from 1 to 12carbon atoms which is unsubstituted, monosubstituted by CN or CF₃ ormonosubstituted to perhalosubstituted by halogen, optionally one or moreCH₂ groups in these radicals being replaced, in each case independentlyof one another, by

 in such a way that O atoms are not linked directly to one another,R^(b) is an alkenyl radical having from 2 to 7 carbon atoms, L¹ is H orF, and x is 0 or
 1. 13. A liquid-crystalline medium according to claim2, which additionally comprises one or more cyano compounds of theformulae IIIa to IIIj:

in which R³ is H, an alkyl radical having from 1 to 12 carbon atomswhich is unsubstituted, monosubstituted by CN or CF₃ or monosubstitutedto perhalosubstituted by halogen, optionally, one or more CH₂ groups inthese radicals being replaced, in each case independently of oneanother, by

 in such a way that O atoms are not linked directly to one another, andL¹, L² and L⁵ are each, independently of one another, H or F.
 14. Theliquid-crystalline medium of claim 13, wherein the medium comprises:one, two or three compounds of each of the formulae A-1 and A-3, two,three or four compounds of the formula B, and two to five compounds ofthe formulae IIIb, IIIc and/or IIIf.
 15. The liquid crystalline mediumof claim 4, which comprises at least one compound of the formula IIIc.16. The liquid-crystalline medium of claim 6, which comprises at leastone tolan compound of the formula T2c.
 17. The liquid-crystalline mediumof claim 1, which comprises at least one compound of the formula A inwhich R^(a) is CH₃, C₂H₅, n-C₃H₇ or n-C₅H₁₁.
 18. The liquid-crystallinemedium of claim 1, which further comprises at least one compound of theformula

in which L¹ is H or F.
 19. A liquid-crystal medium according to claim 1,wherein the medium comprises a total of from three to six compounds ofthe formulae A or B, where the proportion of these compounds withrespect to the mixture as a whole is from 25 to 65% by weight.
 20. Aliquid-crystal medium according to claim 1, wherein the medium has adielectric anisotropy, Δε, of ≧3.